The conventional hydraulic power assist steering system comprises a hydraulic actuator for moving the steering linkage in relation to the fluid flow supplied thereto, and a rotary hydraulic control valve assembly for controlling fluid flow to the actuator in relation to the operator exerted steering torque. The control valve generally includes a cylindrical valve body rotatable within the valve housing, and a spool rotatably disposed within the valve body. Hydraulic fluid is supplied to a cavity formed in the spool, and the valve body is grooved to receive fluid flow in relation to the amount of relative rotation between spool and valve body. The fluid so received is then directed to the actuator so that steering assist is developed in relation to the relative rotation of the valve body and spool.
The spool is manually rotated by the operator of the vehicle and is connected to mechanically drive the steering linkage through a lost motion coupling. A resilient element, such as a torsion bar, couples the spool and valve body to provide a centering force for aligning the spool and valve body, and to permit relative rotation therebetween in relation to operator exerted steering torque, at least within the limitations of the lost motion coupling.
In systems of the type described above, the level of driver steering effort assist required to produce a given level of power assist, depends primarily on the compliance of the torsion bar. If the torsion bar has relatively high compliance, a relatively low level of driver steering effort is required. This is generally desirable in low speed operation of a vehicle where relatively high steering forces are required. If the torsion bar has relatively low compliance, a relatively high level of driver steering effort is required. This is generally desirable in high speed operation of a vehicle where relatively low steering forces are required.
The need to accommodate different steering levels at different speeds has been met by the U.S. Pat. No. 4,871,040 issued Oct. 3, 1989, to Zuraski et al. entitled "Electromagnetic Control Apparatus For Varying The Driver Steering Effort Of A Hydraulic Power Steering System". That patent describes a hydraulic power assist steering system having conventional relatively rotatable spool and valve body elements for flow regulation, and an integral electromagnetic mechanism which defines a coupling of variable resilience between the spool and valve body for adjusting driver steering effort required to produce a given level of power assist. Other solutions for controllability of steering effort are shown in U.S. Pat. No. 4,886;137 issued Dec. 12, 1989, to Pawlak et al. and U.S. Pat. No. 4,886,138 issued Dec. 12, 1989, to Graber et al. Each of these patents teach the use of an electromagnetic mechanism for defining a coupling between the spool and valve body.
In each case, the steering effort can be controlled to a fine degree to achieve design objectives, but is subject to the effects of temperature variability on the system response. Since hydraulic viscosity, properties of magnetic materials, properties of electrical components and mechanical dimensions are all subject to changes with temperature, these characteristics have to be taken into consideration in order to provide adequate compensation for a given temperature and to make steering assist performance uniform at all temperatures to which the steering system is exposed. Although the problem and the solution of this invention apply equally to each of the patents mentioned above, the structure of U.S. Pat. No. 4,871,040 is used herein to describe the invention.
The integral electromagnetic mechanism includes a rotary magnetic circuit and a stationary electromagnetic circuit. The rotary magnetic circuit comprises a pair of relatively rotatable elements, one of which is toothed to conduct magnetic flux and one of which includes permanent magnets for establishing a permanent magnet coupling.
In the illustrated embodiment, the toothed element is defined by a pair of axially displaced magnetic pole pieces, and the permanent magnet element is defined by a disk element disposed between the magnetic pole pieces. The disk element is supported for rotation with the input (operator driven) steering shaft, and the pole pieces are supported for rotation with the output (pinion) steering shaft. The disk element is axially magnetized to define an even number N of radially extending, alternating magnetic polarity sectors. The rotary pole pieces each have N/2 teeth extending toward the respective axial face of the disk element. The stationary electromagnetic circuit comprises at least one annular exciting coil disposed about the rotary magnetic circuit and ferromagnetic pole elements positioned adjacent the rotary magnetic pole pieces.
The above elements define two magnetic flux paths: a permanent magnet flux path which includes (neglecting leakage flux) only the rotary magnetic circuit elements, and an electromagnetic flux path which includes both the stationary and rotary magnetic circuit elements. The rotary pole pieces and the disk element are oriented such that (1) when the assembly is in the centered position, both flux paths are magnetically balanced, and (2) when there is relative rotation of the input and output steering shafts, the flux in the two paths develop in-phase centering forces which tend to restore the assembly to the centered position. The force due to the electromagnetic flux path is variable over a wide range, depending on the magnitude and direction of current supplied to the coil, and such current may be scheduled in relation to the vehicle speed to provide a speed-dependent relationship between the operator input torque and the power assist torque.
In the illustrated embodiment, the mechanism of this invention is used in combination with a conventional torsion bar to define a variable resiliency coupling between the hydraulic fluid supply elements. The combination of the torsion bar and the permanent magnet flux path provide a coupling of intermediate resilience to generate an intermediate level of steering assist for a given driver steering input. Variably energizing the exciting coil with current of one polarity variably increases the resilience of the coupling so that more driver steering effort is required to produce a given level of power assist. Variably energizing the exciting coil with current of the opposite polarity variably decreases the resilience of the coupling so that less driver steering effort is required to produce a given level of power assist. Preferably, the coil energization is scheduled in relation to the speed of the vehicle so that the level of steering assist decreases with increasing vehicle speed. As will be seen below, control of the coil energization is used to carry out compensation for temperature effects on the system. A driver preference input may also be used.